Overbore Velocity

Dear Technoid,

I was wondering if you know what effect overbored barrels have on velocity. I have heard stuff about the wad not sealing correctly, and gas getting around it, and then the gun loses velocity. I have also heard that less friction increases velocity. I shoot 3.5″ steel loads, so a backbored barrel, in my opinion, does help the pattern. I have a Mossberg 835 with a .775 inch barrel (it is a 12 gauge) and I was wondering if there was a risk of the wad not sealing correctly. It has extremely tight patterns with a full choke and handles quite well. I am also looking at buying a Rem. 870, but their barrels are not backbored. Would I get a higher, lower, or similar velocity from both guns?


Dear George,

The possibility of velocity change due to overboring depends on a number of variables. It’s not cut and dried. By the way, “Overboring” is when it is done by the factory at time of manufacture. “Backboring” is when it is done subsequent to manufacture.

The larger the bore of the gun, the greater the chance that the plastic wad will not properly obturate (seal) and that velocity will be affected. Since there is a wide variety of wads in use, the chances of proper wad obturation vary with the bore and with the brand of wad.

Obturation can also vary with the temperature. The plastic used in wad manufacture is a balance of a number of factors. The ability to remain flexible in cold temperatures is only one of them. There’s also cost and “moldability”, plus design. Most plastics become more rigid as they become colder. Rigid wads don’t seal powder gasses as well as flexible ones.

So, consistent velocity can depend on the amount of the overbore, the design of the wad, the plastic used in the wad and the temperature. I’ve shot one of the Baker “Big Bore” ultimate backbore barrels with an .800″ bore. That is the size of the standard 12 gauge chamber, so this barrel was basically a 30″ chamber. You can’t have a 12 gauge bore larger than that unless your forcing cones open UP instead of the usual constriction down. For all intents and purposes .800″ is the largest bore you can use on a 12 gauge gun.

I noticed that even with high quality target ammunition, I sometimes got “off sounding” shots when shooting the Baker Big Bore barrel in the winter. Being slothful, I leave my shells in the trunk of my car, so the shells had been in a cold sink below freezing when they were used. In my experience, an “off sounding” shot is the product of gas blowby and a loss of velocity, though there is often enough speed to break a close target.

I’ve also had “off sounding” shots from standard barrels on warm days. These have been with the Winchester “Feather” low noise/low recoil shells. Obviously, if these shells are marginal in tight bores on warm days, they would have severe problems in overbore barrels on cold days.

As an aside, under any circumstances I try to treat my shells the same way I treat my feet- Keep ’em warm! Sometimes it doesn’t matter, but sometimes it really does. That’s a good rule of thumb. Keep your hunting shells near your body so they don’t freeze. Keep your target shells in the club house or in the cabin of the car as much as possible.

While overboring/backboring has gotten a lot of hype in the past few years as machine shops look for ways to make money and manufacturers look for “something new”, there is a definite downside to big bores. The bottom line is: the larger the bore, the greater the chance of an imperfect gas seal. For years and years Italian guns have had tight bores (.721″ was virtually standard on Berettas for just about ever) for a reason. You get greater velocity consistency that way with the broadest variety of shells.

With modern shells and plastic wads, there’s more room to enlarge the bore. Browning (Japan) feels that something in the .740″ is about as large as they can get away with. Krieghoff has always been around .735″. Mossberg’s .775″ is definitely getting up there.

There is the theory (espoused by those who machine or sell overbore barrels) that the overbore reduces friction and thus increases velocity. I’m sure it does. Baker claimed an extra 50 fps for his Big Bores. An extra 50 fps until you got a blooper, that is. I tested Little Skeeter (www.littleskeeters.com) chamber inserts for a little magazine article some time ago. These are just subgauge chamber inserts. They are essentially the hindmost 2-3/4″ of a skeet tube. Another way to look at them is as an aluminum 12 gauge shell drilled out to accept a 20, 28 or 410 shell. They totally end before the forcing cone and have absolutely no barrel at all. You simply can’t get anything shorter.

Yet, in the 20 and 28 gauge inserts in the 12 gauge barrel, the velocities I chronographed were relatively normal and fairly consistent. I don’t know what is happening in that 12 gauge bore, but I doubt that a 28 gauge wad meant for a .515″ bore can expand to seal in a .729″ bore. The article goes into more detail and I don’t want to give it all away here, but my guess is that lack of friction has a lot to do with it. Using the Little Skeeters was like watching a dog walk on its hind legs. You don’t really comment on how well it walks. You are just amazed that it can walk at all. The performance of the Little Skeeters has definitely made me rethink my thoughts of overbored barrels.

Bottom line: I don’t have a hard and fast answer. I still believe that tight bores offer the best chance of a proper wad seal under all conditions and with the largest number of wads. Tight bores may well have a greater effect on shot setback of LEAD shot (not steel) and thus produce patterns technically inferior to larger bores, but there are so many other factors involved in patterns that this is only one variable.

I simply don’t know what comparable velocities you will get using 3.5″ steel loads in a Mossberg 835 vs a Remington 870. I don’t own a Mossberg (though I have nothing at all against them) and I don’t shoot 3.5″ steel.

If you are convinced that an overbore barrel helps your patterns from 3.5″ steel, then stay with the Mossberg just as long as you haven’t noticed any “off sounding” shells. If they sound right, they probably are right, or close enough. The only way you are ever going to know for sure is to chill your shells down to the temperatures at which you hunt and then chronograph them. That’s your job, not mine.

If you are just dying to get a new 870, then possible velocity variation with the overbore barrels of the Mossberg is as good an excuse as any for a new gun. There. That’s what you really wanted to hear, isn’t it?

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)

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